It is common practice to obtain a metal shaft having a partly increased diameter by machining a blank shaft of a relatively large diameter. However, this machining process takes time, and what is worse, wastes metal in the form of cutting chips.
In general, the mechanical power transmission shafts require components such as gears, cams, and sprockets whose diameter is larger than that of the shafts. In order to provide the metal shafts with these components, a mechanical method is not economical where the metal flesh of a shaft is machined to form gears as integral parts. An alternative way is to produce those component parts on a separate process, and then join them to the shafts by welding or bolting. This method is not efficient. Therefore, a metallurgical process was proposed for forcing a metal shaft to diametrically expand in a desired portion, and cutting gears or cams there. However, it has been considered to be impracticable to put the proposed metallurgical method in practice.
The inventor of the present application invented a method of expanding the diameter of a metal shaft in its middle portion through rotation, bending and compression, and has obtained Japanese Patent No. 1,993,956. This metallurgical method has overshadowed the conventional mechanical method, and made it possible to form gears or cams in the diametrically expanded portion of a metal shaft.
Nevertheless, the patented method is at the experimental stage, and is not fully developed for mass-production basis. The present invention has overcome the obstacles to practical use.